During the treatment of granular solids such as sulfidic zinc ore, iron ore, sponge iron or aluminumhydroxide, it is the object in many regions that the solids inventory, for example, the quantity and hence the vertical level of the solids in a fluidized bed, must be kept constant. There are various solutions to this object. On the one hand, a so-called aperture blocker or a discharge lance can be used. This is a mechanical solids valve in the form of a lance with a conical tip which fits into a corresponding conically shaped opening of the tank wall of the fluidized bed. By withdrawing or inserting the lance into the opening, the cross-section is increased or reduced, so that the outflow can be controlled. However, the same pressure exists on both sides of the solids outlet, because the aperture blocker can effect a pressure seal only in the completely closed condition. In general, this will be the pressure of the fluidized bed at the level of the solids outlet. If as a result of the switching of the process and/or the respective operating condition a differential pressure is obtained over the solids outlet, a deterioration of the control quality must be expected.
EP 0 488 433 B1 describes a control aperture blocker for opening and closing a gas passage is described in detail.
Such control aperture blockers function in practice, but they have their weaknesses and disadvantages. On the one hand, the control aperture blocker has mechanically moving parts, which are in contact with the solids. Therefore, it must be cooled by water cooling if the solids are hot. The flow rate of the cooling water and the temperature difference between forward flow and return flow must be monitored. Occasionally, a damage of the lance occurs. Water then escapes from the lance and, in the worst case, flows into the tank located below the same, which has a refractory lining, whereby said refractory lining can be damaged. In addition, the lance must be moved laterally, with the drive being located on the outside at ambient pressure, and excess pressure or negative pressure typically existing inside. For sealing purposes, a stuffing box is used. If this becomes leaky, hot solids probably will be discharged, which represents a safety hazard, or ambient air will enter, which can disturb the process. To adjust the stream of solids discharged via the aperture blocker, an exact adjustment is required between the tip of the lance and the nozzle stone acting like a valve seat. It should be considered here that after extended operating periods the high temperatures can effect a displacement of the refractory lining, so that this exact adjustment can get lost. It can also occur that after an extended period with closed aperture blocker, the solids are defluidized before the tip of the aperture blocker and do not move upon opening the aperture blocker. In many cases, a manually handled air lance, which is moved through another stuffing box, can then be used for poking and at the same time fluidizing the solids. The success or failure of such poking typically can be observed through an inspection glass. When the solids are hot enough to glow, something can be seen. But if they are cold, nothing can be seen and one is working blind, so to speak. In the case of hot solids, however, the inspection glass withstanding the high temperatures is very expensive. Moreover, with a control aperture blocker, a pressure seal cannot be realized via the control device. This can lead to gas/air flows through the nozzle stone, in the worst case also against the direction of the solids flow, whereby the solids flow can be limited or even be inhibited completely.
Alternatively, the level of the inventory in the fluidized bed can also be kept constant by a weir or a discharge opening disposed at a firmly defined distance from the distributor plate. This is frequently employed in stationary or bubble-forming fluidized beds. When the fluidized bed has a higher or lower pressure than the surroundings or a succeeding tank into which the solids flow, a pressure seal must yet be realized. For this purpose, so-called float chambers, siphons or star feeders can be employed.
A siphon for conveying fine-grained solids is described in DE 196 29 289 A1. The siphon consists of a conduit connected with a means for feeding the solids and an almost oppositely directed second conduit to which a further conduit is connected, in which the solids are conveyed under the influence of gravity. Into a region filled with fine-grained solids, a lance extends for fluidizing the solids. The apparatus is used for blocking the pressure in a fluidized bed with respect to the underflow of a recirculation cyclone and for recirculating the solids from the cyclone back to the fluidized bed. A specific control of the stream of solids is not possible. The supply of air merely serves to keep the solids in a fluid-like condition. In such a system, the solids level in the fluidized bed is not variable.
In the case of star feeders, the stream of solids and hence the solids level in the fluidized bed can be influenced by varying the rotational speed, and when new, a pressure seal can also be achieved. However, they are disadvantageous in so far as the rotating rotor is in direct contact with the solids, whereby wear occurs and tightness is endangered. In addition, the shaft of the rotating rotor must be sealed against the surroundings, because the drive is arranged on the outside.
A further disadvantage of all the aforementioned systems consists in that they only work in a downward direction, for example, the solids arrive at a level lower than the level in the fluidized bed.
U.S. Pat. No. 6,666,629 describes a method for conveying granular solids, by means of which height can also in principle be overcome. By means of a gaseous medium, the solids are conveyed from a first zone with a pressure of 4 to 16 bar through a descending conduit and via an ascending conduit to a second zone with a pressure lower than in the first zone by 3 to 15 bar. The inflow of the gaseous medium is effected through an upwardly directed nozzle at the point where the descending conduit opens into the ascending conduit. Moreover, additional gas is introduced into the descending conduit, which determines the stream of solids through the descending conduit.
WO 01/28900 A1 describes an apparatus in which solids are conducted through a downer to an ascending conduit, through which they are conveyed by means of fluidizing gas and then are withdrawn at the bottom upon deflection. By means of numerous gas supply conduits, the stream of solids is fluidized both in the descending conduit and in the riser along the entire length and is thereby conveyed by gravity like a fluid in communicating tubes.
US 2005/0058516 A1 describes an apparatus for the transport of fine-grained solids with a controlled flow rate, wherein the solids initially flow downwards through a downer as a result of gravity and then are transported to a riser via an inclined transfer conduit by injecting a secondary gas, in which riser air is introduced from below, in order to convey the particles to the top. The downer and the riser accordingly are not directly connected with each other. In the connecting piece, the solids are fluidized and supplied with secondary air. The conveying air in the riser is kept constant, whereas the control of the stream of solids is effected via the secondary air in the connecting piece.
The stream of solids discharged via the riser then can be supplied to a mixing tank in which it is mixed with another stream of solids. Such mixture, however, without use of a riser formed as described above, is described, for instance, in DE 195 42 309 A1. When producing alumina from aluminum hydroxide, a partial stream of the predried and only slightly preheated hydrate is passed by the furnace of a calcining plant and then mixed with the hot alumina from the furnace of the calcining plant. It is difficult, however, to precisely define the temperature in the mixing tank and the mixing ratio of alumina and aluminum hydroxide, as the mass flows can hardly be measured. This is why in practice a speed-variable star feeder mostly is used for the hydrate passed by the furnace, by means of which the temperature in the mixing tank is controlled. However, this involves the above-described typical disadvantages of the star feeder such as wear and decreasing tightness, so that a reliable pressure seal is almost impossible.